Composition and Accretion of the Terrestrial Planets

Lunar and Planetary Science XXXI 1546.pdf

COMPOSITION AND ACCRETION OF THE TERRESTRIAL PLANETS. Edward R. D. Scott and G. Jeffrey Taylor, Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, Univer- sity of Hawai’i at Manoa, Honolulu, Hawai’i 96822, USA; [email protected]

Abstract: Compositional variations among the spread gravitational mixing of the embryos and their four terrestrial planets are generally attributed to giant 20 impacts [1] rather than to primordial chemical varia- Fig. 2 tions among planetesimals [e.g., 2]. This is largely be- Mars cause modeling suggests that each terrestrial planet ac- 15 creted material from the whole of the inner solar system [1], and because Mercury’s high density is attrib- 10 Venus Earth uted to mantle stripping in a giant impact [3] and not to its position as the innermost planet [4]. However, Mer- Mercury cury’s high concentration of metallic iron and low con- 5 centration of oxidized iron are comparable to those in recently discovered metal-rich chondrites [5-7]. Since E 0 chondrites are linked isotopically with the Earth, we 0 0.5 1 1.5 2 suggest that Mercury may have formed from metal-rich chondritic material. Venus and Earth have similar con- Semi-major Axis (AU) centrations of metallic and oxidized iron that are inter- fragments, which ensured that each terrestrial planet mediate between those of Mercury and Mars consistent formed from material originally located throughout the with wide, overlapping accretion zones [1]. However, inner solar system (0.5 to 2.5 AU). Thus any initial Mercury and Mars, which have a combined mass of radial chemical variations in the compositions of the only 0.16 Earth masses, may represent planetary em- planetesimals would be largely erased as the planets bryos that accreted from narrow zones of planetesimals. accreted. Wetherill [1] found no obvious tendency for The inverse correlation between the concentrations of Mercury-like planets to be derived from material ini- oxidized and metallic iron among the terrestrial planets tially located at the inner edge of the disk of embryos. may be a primordial feature of the inner solar nebula However, these models appear incompatible with the and other planetary systems with Earth-like planets. inferred mantle FeO concentrations of the planets, espe- cially those for Earth and Mars (Fig. 2). 5.5 Subsequent modeling by Chambers and Wetherill Mercury Fig. 1 [12] confirmed that Earth- and Venus-like planets ac- 5 crete from broad zones. However, their models suggest that Mars may be a single embryo, i.e., a body that 4.5 formed during earlier runaway growth when orbits were Venus nearly circular and coplanar. Thus Mars may be de- Earth rived from a restricted part of the nebula that was poorly 4 Mars sampled by the other planets. Accretion models for Mercury are critically dependent on the assumed initial 3.5 conditions and the reason for its small size. We infer that Mercury may be small and chemically distinctive 3 because it also formed from a single embryo. 0 0.5 1 1.5 2 Metal-rich chondrites: The discovery of three new Semi-major Axis (AU) chondrite groups (CR, CH and Bencubbin-like) among Antarctic meteorites has greatly expanded the compositional range of chondrites [5-7. 13, 14]. The new chon- Composition of the terrestrial planets: The un- drites have normal levels of refractory elements but are compressed densities of the terrestrial planets [8] are richer in metallic Fe (some were classed as iron meteor- inversely correlated with semi-major axis (Fig. 1). Each ites) and poorer in volatile elements like Na, K, and S planet except Mars has a density that is higher than the than other chondrites. Bencubbin-like chondrites have densities of the well studied C, O and E chondrite more metal (~80 wt.%) than Mercury m(~70 wt.%) and -3 groups (£3.7 g cm ). Mantle FeO concentrations of the comparable FeO concentrations. Overall, the chondrites terrestrial planets [9-11] appear to be positively corre- show an inverse correlation between metal and FeO lated with semi-major axis (Fig. 2). concentration (Fig. 3), like the terrestrial planets (Figs, Width of planetary accretion zones: Wetherill [1] 1 and 2). modeled the accretion of the terrestrial planets from a Did Mercury form in a giant impact? Most au- disk of lunar to Mars-sized embryos. He found wide- thors have embraced the idea that Mercury’s high con- Lunar and Planetary Science XXXI 1546.pdf

PLANETARY COMPOSITIONS: E. R. D. Scott and G. J. Taylor planetesimals were more important than giant impacts 50 in establishing planetary compositions. However, the CI Fig. 3 nebular processes responsible for diverse chondritic 40 CM compositions are not clear. The low volatile abun- R, CV Chondrite dances of metal-rich chondrites are consistent with CO Groups Lewis’ equilibrium nebula condensation theory [2] and 30 CK some metal grains have compositions consistent with LL CR condensation above 1200 K [18]. However, these chondrites could not have accreted above 1200 K, as Lewis 20 L inferred for Mercury, because their metal grains were H rapidly cooled in days or weeks to below 600 K [19]. Rapid condensation of metal and silicate in localized 10 CH nebular processes may have triggered accretion of Ben planetesimals. Thus Mercury may be small and metal- EH, EL 0 rich because only a small fraction of the metal and an 0 25 50 75 100 even smaller fraction of the silicate condensed and ac- metal wt% creted . The remainder of the metal and silicate probably accompanied the volatiles into the Sun.

centration of metal is a result of a giant impact that Implications: 1) M-type asteroids with 3-mm H2O preferentially removed most of the silicate mantle [3]. bands [21] might be made of matrix-bearing, metal-rich However, successful models required extreme condi- chondrites like CH chondrites. 2) The metal-rich plan- tions: a) targets containing 32 wt.% metallic iron; b) ets, Mercury, Earth and Venus, may have formed pref- head on collisions at 20 km/s or oblique impacts at im- erentially from metal-rich chondritic material. Mercury plausibly high collision speeds of 35 km/s [15]; c) re- may have acquired its large core from planetesimals moval of all silicate ejecta as sub-centimeter-sized parti- containing 70 wt.% metallic iron, not from a giant im- cles by the Poynting-Robertson effect to prevent sili- pact. 3) Correlations between uncompressed density, cates from reaccreting on to Mercury [3]. In addition, mantle FeO concentration and semi-major axis (Figs. 1- the impact model does not account for the unusually low 2) may reflect primordial compositional trends that are FeO concentration of Mercury. present in chondrites (Fig. 3). 4) If the Moon reflects Constraints on planetary compositions from the composition of the large impactor, its mantle FeO chondrites: Wasson’s claim that E chondrites match concentration of 11 wt. % [10] suggests an origin be- the composition of planetesimals that formed near 1 AU tween Earth and Mars. 53 52 [15] is supported by Cr/ Cr data in planetary samples References: [1] Wetherill G.W. (1994) GCA 58, [16]. Either E chondrites were transported to the aster- 4513-4520. [2] Lewis J. S. (1972) EPSL 15, 286-290. oid belt from ~1 AU soon after they formed [15] or they [3] Benz W. et al. (1988) Icarus 74, 516-528. [4] Taylor are rare samples that formed in the asteroid belt when S. R. (1991) Meteoritics 26, 267-277. [5] Weisberg M. planetesimals near 1 AU were accreting. Several argu- K. (1999) LPS 30, #1416. [6] Scott E. R. D. (1988) ments suggest that the metal-rich chondrite groups (CH EPSL 91, 1-18. [7] Grossman J. N. et al. (1988) EPSL and Bencubbin-like) may also resemble materials that 91, 33-54. [8] Kaula W. M. (1986) in The Solar System accreted in the inner part of the solar nebula. Chon- ed. M. G. Kivelson, pp. 78-93. [9] Longhi J. et al. drites like CI and CM that have approximately solar (1992) In Mars ed. H. H. Kieffer et al., pp. 184-208. compositions and contain abundant hydrated matrix [10] Taylor S. R. (1992) Solar System Evolution, Cam- material probably formed furthest from the Sun. Metal- bridge. [11] Robinson M. S. et al. (1997) LPS 28, 1189- rich chondrites, which are highly deficient in matrix 1190. [12] Chambers J. E. and Wetherill G. W. (1998) and volatile elements, probably formed closest to the Icarus 136, 304-327. [13] Weisberg M. K. et al. (1993) Sun. Chondrule-type is also correlated with matrix GCA 57, 1567-1586. [14] Weisberg M. K. et al. (1995) abundance and hence provides another guide to forma- Proc. NIPR Symp. 8, 11-32. [15] Wasson J. T. (1988) In tion location [17]. Metal-rich chondrites contain abun- Mercury ed. F. Vilas et al., pp.622-650. [16] Shu- dant cryptocrystalline chondrules which are characteris- kolyukov A. and Lugmair G. W. (1999) LPS 30, # 1093. tic of higher formation temperatures and less common [17] Rubin A. E. and Wasson J. T. (1995) MAPS 30, in C and O chondrites. Thus chondrite trends predict 569. [18] Meibom A. et al. (1999) JGR 104, 22,053- that the innermost planets formed from metal-rich 22,059. [19] Meibom A. et al. LPS 30, # 1411. [20] chondritic planetesimals. Jones T. D. et al. (1990) Icarus 88, 172-192. Acknowl- Why do terrestrial planets have diverse composi- edgements: We thank A. Rubin, J. Wasson, K. Keil, A. tions? Since the variations in metallic and oxidized Meibom and S. Krot for helpful discussions. This work iron in the planets mimic those in chondrites, we pro- was partly supported by NASA grant NAG 5-4212 to K. pose that primary chemical differences among Keil.